Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for making a seismic survey and, more particularly, to mechanisms and techniques for detecting the presence of marine mammals and mitigating their exposure to undesired sound levels generated by a seismic source.
Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of geophysical structures under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where the oil and gas reservoirs are located.
For marine acquisition, a seismic acquisition system 100 includes, as illustrated in FIG. 1, a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source array 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to the head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to the tail end 1108 of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth. Seismic sensors 122 are distributed along the streamer and configured to record seismic data. Seismic sensors 122 may include a hydrophone, geophone, accelerometer or a combination thereof. Positioning devices 128 (also known as birds) are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which are typically air guns. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. The source elements attached to float 137 form a sub-array. Source array 130 may have multiple sub-arrays, typically 3. Traditionally, vessel 102 tows two source arrays 130 and 130′, which may be identical. During operation, vessel 102 follows a predetermined path T while source elements (usually air guns) 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144, which are recorded by sensors 122. The positions of both source elements 136 and recording sensors 122 may be estimated based on GPS systems 124 and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even fully process the data. Controller 126 may also be connected to the vessel's navigation system and other elements of the seismic survey system, e.g., positioning devices 128.
When a conventional marine source (e.g., air gun) is fired, it generates a powerful underwater air bubble that oscillates for a couple of seconds, thus generating sound waves. These waves propagate through the water toward the ocean bottom as illustrated in FIG. 1. However, some of the energy associated with this bubble propagates as a sound wave 150, substantially parallel to the water surface 104, with the potential of interfering with activities of a marine mammal 152. To prevent this kind of situations, the seismic surveys are regulated by various national and international bodies that require that marine mammal activity is monitored and protected from harmful sound waves.
More specifically, international and national regulations in this regard and the recommendations of competent bodies encourage marine seismic operators to carry out, in parallel to their measurement campaign, continuous monitoring for detecting the possible presence of marine mammals in the exploration zone.
This monitoring is currently carried out either visually by one or more human operators and/or using an independent system of acoustic receiver or receivers fulfilling functions of passive detection, classification and geographical location of the marine mammal or mammals. In this regard, U.S. Pat. Nos. 7,974,151 and 8,937,847, the entire content of which is incorporated herein by reference, disclose such mammals detection systems. Once a marine mammal is detected within an exclusion zone centered on the seismic source, current regulations require that the seismic source is stopped.
These acoustic systems 160, commonly referred to as Passive Acoustic Monitoring (PAM), are also towed by the streamer vessel 102 as illustrated in FIG. 1. PAM system 160, which is shown in FIG. 2 in more detail, includes acoustic receivers 122′ distributed along one or more antennas 110′ dedicated to this function. A telemetry system (not shown) transmits the data recorded by sensors 122′ to a remote controller (i.e., processing unit) 126, located for example on the seismic vessel. Alternatively, mammal sensors 122′ may be integrated with seismic sensors 122 into the same streamer 110 as shown in FIG. 2. In still another embodiment, mammal sensors 122′ may be integrated into seismic sensors 122, i.e., the same sensor is used for both seismic activity and mammal detection functions as also shown in FIG. 2. In other words, the existing PAM systems are deployed together with in parallel to the seismic streamers.
A PAM system may also include equipment for “acoustic deterring” of marine mammals. This equipment emits artificial acoustic signals in order to drive away mammals before the implementation of activities assumed to be dangerous for these species. Such equipment is described in U.S. Pat. No. 8,937,847.
When a mammal is detected by the PAM system, current practices involve stopping the firing of the seismic source until the mammal has left the so called exclusion zone. The exclusion zone is a volume (typically a sphere) centered on the seismic source and having a given radius. The presence of a marine mammal inside the exclusion zone is considered to be dangerous for the mammal, and for this reason, the firing of the source is stopped when the mammal is detected inside the exclusion zone.
However, such a practice has the following possible disadvantages: it does not guarantee that a marine mammal is protected from the sound waves associated with the seismic source, slows down the data acquisition, which increases the costs of the survey, and requires quick human intervention. Besides, the existing systems are not pro-active, i.e., they do not anticipate the mammals' presence, rather they are reactive, i.e., they detect the mammals' presence only when the mammals are so close to the acquisition system that not many choices are left for the vessel's operator. This needs highly competent/qualified technicians for quickly and adequately reacting to the detected mammal.
Therefore, there is a need to improve or come up with a new PAM system that avoids the above noted disadvantages, and especially protects the marine mammals.